Methods of utilizing photoresists which are sensitive to ultraviolet rays or visible rays have been conventionally widely put to practical use in pattern forming for producing electronic parts such as semiconductor elements, magnetic bubble memories, integrated circuits, etc. Photoresists are classified into two types of a negative type in which the part irradiated with light becomes insoluble in a developing solution, and a positive type in which the irradiated part becomes soluble in a developing solution contrary to the negative type. As the negative type photoresist is excellent in sensitivity, the adhesive property with substrates requisite in wet etching and chemical resistance as compared with the positive type, the negative type has stood first in the photoresist until recently.
However, the line widths and intervals of resist patterns have become extremely fine with the trend of higher density and higher integration of semiconductor elements, etc., and dry etching has come to be adopted in the etching of substrates, as a result, the photoresist is required to have higher resolving degree and higher resistance against dry etching. For these reasons, the positive photoresist accounts for mainstream at present. In particular, because of high sensitivity, high resolving degree and excellent dry etching resistance, among the positive photoresists, alkali development type positive photoresists based on alkali-soluble novolak resins described, for example, in J. C. Strieter, Kodak Microelectronics Seminar Proceedings, p. 116 (1976) have now become mainstream.
With the tendency of multifunction and high advancement of electronic equipments in recent years, however, refinement of patterns has been eagerly demanded for further increase in density and integration.
As the dimension of the IC in the machine direction cannot be so reduced as compared with the reduction in the transverse direction, the ratio of the height to the width of the resist pattern cannot be helped to become larger. Therefore, it has become more and more difficult to suppress dimensional changes of resist patterns on the wafers having complicated structural differences in level with the progress of refinement of patterns.
Further, problems have also arisen in various exposing methods with the contraction of the lower limit. For example, in exposure with a light, the interference action of the reflected light due to difference in level of the substrate largely affects dimensional accuracy. On the other hand, in exposure with an electron beam, the ratio of the height to the width of a refined resist pattern cannot be increased by the proximity effect caused by the back scattering of the electron.
It has been found that these many problems can be resolved by employing a multilayer resist system. With respect to a multilayer resist system, a general statement is described in Solid State Technology, 74 (1981), and other many studies have been disclosed as to the multilayer resist system.
In general, the multilayer resist system is classified into a three-layer resist system and a two-layer resist system. The three-layer resist system is a system comprising coating an organic flattened layer on a substrate with differences in level, laminating an inorganic interlayer and a resist thereon, patterning the resist, dry etching the inorganic interlayer with making the resist a mask, and patterning an organic flattened layer by O.sub.2 RIE (reactive ion etching) with making the inorganic interlayer a mask. This system has been discussed from early as conventional techniques can be fundamentally used but there arise such problems that the process is very complicated, or cracks and pinholes are liable to be generated in the interlayer because three layers, i.e., an organic layer, an inorganic layer, an organic layer, each having different physical properties are superposed.
Contrary to the three-layer resist system, a resist having both properties of the resist and the inorganic interlayer in the three-layer resist system, i.e., a resist having oxygen plasma resistance, is used in the two-layer resist system, so that generation of cracks and pinholes can be prevented and, further, process is simplified from three layers to two layers. However, contrary to the three-layer resist system wherein conventional resists can be used, there is such a subject in the two-layer resist system that a novel resist having oxygen plasma resistance has to be developed.
From the above-described background, the development of a positive photoresist which can be used as the upper layer resist in the two-layer resist system and having excellent oxygen plasma resistance, high sensitivity and high resolution, in particular, an alkali development type positive photoresist which can be used without modifying the present process has been desired.
For the above development, various photosensitive compositions have been suggested such as polysiloxanes comprising conventional orthoquinonediazides provided with alkali solubility, or photosensitive compositions comprising silicon polymers such as polysilmethylene, etc., in combination, e.g., photosensitive compositions disclosed in JP-A-61-144639 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-61-256347, JP-A-62-159141, JP-A -62-191849, JP-A-62-220949, JP-A-62-229136, JP-A-63-90534, JP -A-63-91654, and U.S. Pat. No. 4,722,881, and photosensitive compositions comprising polysiloxane/carbonate block copolymers combined with an effective amount of onium salts disclosed in JP-A-62-136638.
However, alkali solubility of any of these silicon polymers is given by the introduction of a phenolic OH group or a silanol group (.ident.Si--OH). When alkali solubility is given by the introduction of a phenolic OH group, the production is extremely difficult, and when alkali solubility is given by a silanol group, the stability with the lapse of time is not necessarily good, or the layer thickness after development extremely decreases and a rectangular shape cannot be obtained.
Photoresists containing a polysiloxane which has in the molecule a group decomposable by the action of an acid are disclosed in U.S. Pat. No. 2,736,939 and JP-A-9-274319. However, these photoresists have various problems that the resolution is low, a rectangular form cannot be formed, and show considerable dimensional shift at the time of pattern transfer to the lower layer in the subsequent oxygen plasma process.